During and following integrated circuit chip manufacturing processes, integrated circuits are inspected to detect defects. The process by which the defects are detected is known as failure analysis. Failure analysis may be accomplished by a wide array of techniques. One technique used to perform failure analysis is to physically test one chip at a time with a probe. The probe provides power to the circuit. The resistive heating of the circuit elements can then be characterized, the difference between good (intended) and bad (unintended) circuits can be detected and problems isolated. However, since many chips may be built-up on a wafer, the process of performing failure analysis on one chip at a time is slow. A technique employed to improve the speed of failure analysis is emission microscopy. Most chip-level defects emit light at particular wavelengths when stimulated by a light source or an electric current, and the emitted light may be quickly collected and analyzed to detect defects. Thus, emission microscopy is an efficient way to detect defects in integrated circuits.
However, the total number of integrated circuit layers built-up on chips has been greatly increasing. Indeed, many chips now comprise so many layers that the light emitted by a defect may never escape from the chip layers if the wafer substrate is too thick. (Other failure analysis techniques, such as thermal analysis, also suffer difficulties because of the large number of layers built up on the wafer.) One solution to this problem has been to grind the backside of the wafer to make the wafer extremely thin, as thin as 50 micrometers (50 microns or 50 μm). Once the wafer is thinned, light emitted from defects in the layers can escape through the backside of the wafer and then be analyzed. Failure analysis using this technique is known as backside emission microscopy.
To increase production efficiency, failure analysis may be performed on an entire wafer containing many chips, as opposed to cutting the wafer into separate chips and analyzing each chip one at a time. However, since the backside of the wafer is extremely thin, the entire wafer is frangible and the circuits built up on the wafer can be easily damaged or broken during failure analysis. The problem is complicated by the fact that chip manufacturers desire to build up chips on wafers of the largest diameter. Larger diameter wafers require more support than smaller diameter wafers.
The problem of thin wafer breakage is further complicated by the fact that physical force may be applied to the wafer during and after failure analysis. For example, a probe may be applied to the front side of the wafer during backside emission microscopy. If the probe applies physical force to the wafer, and if the wafer is not adequately supported, then the wafer or the circuits built on the wafer may be damaged. Thus, wafers need support during failure analysis to avoid unnecessary damage to wafers and to the chips built-up on them.
One support technique that has been used during failure analysis is to glue the wafer to a rigid optical windowpane using an optical adhesive. Backside emission microscopy is then conducted through the window pane. A problem with gluing the wafer to the windowpane is that in many techniques the glue is not removable, thereby permanently and undesirably attaching the windowpane to the wafer. Even if the glue is removable, the process of removing the wafer from the windowpane sometimes damages the wafer. Thus, improved devices and methods are needed to support thin wafers (wafers of a thickness below about 250 μm) during failure analysis.
Devices have been proposed to support thin wafers during other wafer or chip manufacturing steps. For example, Goodman et al., Low Mass Wafer Support System, U.S. Pat. No. 6,454,865 (Sep. 24, 2002) shows a wafer support ring for use in high temperature ovens. A first shelf extends from the inside diameter of the support ring and a second shelf extends from the first shelf. A passage in the support ring allows gas to flow between a wafer, which rests on the first shelf, and a base plate, which rests on the second shelf. Bracher, Wafer Support, U.S. Pat. No. 4,182,265 (Jan. 8, 1980) also shows a holding ring for supporting the edge of a wafer.